SUMMARY

The effect of CO2 and light on NH4+ assimilation by Cyanidium caldarium, a thermophilic acidophilic unicellular alga isolated in volcanic areas in Yellowstone National Park, USA, was investigated. N-sufficient cells assimilated NH4+ at a rate of 189 μ 4.76 umol ml-1 packed cell volume (pcv) h-1. Removal of CO2 or darkening almost immediately prevented NH4+ assimilation. N-limited cells in light assimilated NH4+ at the rate of 498 ± 8.05 pmol ml-1 pcv h-1 in the presence of CO2. In darkness they assimilated NH4+ at a rate of 303 ± 1.5 μmol ml-1 pcv h-1 in the presence of CO2, which was as high as 60% of assimilation in the light, and at a similar rate in the absence of CO2. However, after 40 min under the latter conditions, assimilation underwent a time dependent inhibition and ceased after 70 min; it was resumed upon resupply of CO2. In the absence of CO2 in light, NH4+ was assimilated at a considerably lower rate than in darkness, which supports the idea that, under CO2-free conditions, a light-dependent inhibition of NH4+ assimilation occurred. These results are consistent with the contention that cells of C. caldarium, grown under excess NH+4, obtain carbon skeletons for NH+4 assimilation exclusively by photosynthetic reactions, thereby the light and CO2 dependence. Cells grown under conditions of N-limitation possess the ability to obtain a consistent amount of additional carbon skeletons from mobilization of carbon reserves.